Textural and micro-analytical insights into mafic–felsic interactions during the Oruanui eruption, Taupo

  • Shane M. Rooyakkers
  • Colin J. N. Wilson
  • C. Ian Schipper
  • Simon J. Barker
  • Aidan S. R. Allan
Original Paper


Quenched juvenile mafic inclusions (enclaves) are an occasional but informative component in the deposits of large felsic eruptions. Typically, the groundmasses of these inclusions rapidly crystallize as the mafic magma is chilled against a more voluminous, cooler felsic host, providing a physical and chemical record of the nature and timing of mafic–felsic interactions. We examine mafic inclusions of two compositional lineages (tholeiitic and calc-alkaline) from deposits of the 25.4 ka Oruanui eruption (Taupo, New Zealand). 2-D quantitative textural data from analysis of back-scattered electron images reveal a marked diversity in the groundmass textures of the inclusions, including median crystal sizes (amphibole: 14–45 µm; plagioclase: 21–75 µm) and aspect ratios (amphibole: 1.7–4.2; plagioclase: 2.1–4.0), area number densities (amphibole: 122–2660 mm−2; plagioclase: 117–2990 mm−2), area fractions (ϕ) of minerals (ϕplag = 23–45%, ϕamph = 0–28%, ϕcpx = 0–6%, ϕoxides = 0.6–5.5%), and the relative abundance of plagioclase and amphibole (ϕplag/ϕamph = 1.0–4.6). Textural parameters vary more significantly within, rather than between, the two compositional lineages, and in some cases show marked variations across individual clasts, implying that each inclusion’s cooling history, rather than bulk composition, was the dominant control on textural development. Groundmass mineral compositions are also diverse both within and between inclusions (e.g. plagioclase from An34–92, with typical intra-clast variability of ~ 20 mol%), and do not correlate with bulk chemistry. Diverse groundmass textures and mineral and glass chemistries are inferred to reflect complex interplay of a range of factors including the degree and rate of undercooling, bulk composition, water content and, possibly, intensive variables. Our data are inconsistent with breakup of a crystallizing ponded mafic layer at the base of the Oruanui melt-dominant body, instead implying that each inclusion partially crystallized as a discrete body with a unique cooling history. Extensive ingestion of mush-derived macro-crystals suggests that mechanical breakup of mafic feeder dikes occurred within a transition zone between the mush and melt-dominant magma body. In this zone, the mush lacked yield strength, as has been inferred from field studies of narrow (meters to few tens of meters) mush-melt transition zones preserved in composite intrusions. Evidence for plastic deformation of inclusions during eruption and the abundance of fresh residual glass in inclusions from all eruptive phases suggest that the inclusions formed syn-eruptively, and must have been formed recurrently at multiple stages throughout the eruption.


Oruanui eruption Taupo Volcanic Zone Quench crystallization Mafic–felsic interaction Taupo volcano Textural analysis 



SMR acknowledges support from a Victoria University MSc scholarship. Australian Synchrotron access was gained from proposal 2015/1-M9095 and supported by the New Zealand Synchrotron Group. CJNW was supported by a Cook fellowship from the Royal Society of New Zealand. We thank John Gamble, Jim Cole, John Stix and Bob Wiebe for helpful comments and discussions, Katie Preece and an anonymous reviewer for constructive reviews that improved the manuscript, and Gordon Moore for editorial handling.

Supplementary material

410_2018_1461_MOESM1_ESM.xlsx (562 kb)
Supplementary material 1 (XLSX 561 KB)


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© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.School of Geography, Environment and Earth SciencesVictoria University of WellingtonWellingtonNew Zealand
  2. 2.Department of Earth and Planetary SciencesMcGill UniversityMontrealCanada
  3. 3.School of EnvironmentUniversity of AucklandAucklandNew Zealand
  4. 4.New Zealand Petroleum and MineralsWellingtonNew Zealand

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